Vehicles traditionally only have heat-engine traction, in which the driving torque is supplied only by a thermal internal combustion engine. In order to reduce polluting emissions and to reduce consumption, hybrid traction is becoming increasingly more popular, in which the driving torque is supplied by a thermal internal combustion engine and/or by an electric machine working as a motor. According to the type of motion of the vehicle and to the residual electric charge existing in the vehicle batteries, either only the thermal internal combustion engine or only the electric machine is used, or both are used in order to optimize fuel consumption and/or minimize polluting emissions. The electric machine may run as a motor, absorbing electricity and generating a mechanical driving torque, or as a generator, absorbing mechanical energy and generating electricity.
A problem which applies to automotive electric machines relates to the disposal of the heat produced by dissipation by the electric machine itself. In particular, while ensuring adequate cooling of the stator of the electric machine is relatively simple, ensuring adequate cooling of the rotor of the electric machine is not as straightforward. In other words, the heat produced in the stator may be easily evacuated by cooling it (e.g. with air or by means of a hydraulic circuit) because there are no problems of reachability, as it is static and arranged outside; on the contrary, in order to be evacuated, the heat produced in the rotor requires a more complex system due to the rotation of the rotor itself, which makes it more difficult to be reached, combined with the arrangement of the rotor inside the stator.
As described in patent applications JP2007228669A and US20050156471A1, for example, in order to cool the rotor of an electric machine, it has been suggested to provide a hollow shaft of the electric machine, i.e. provided with a longitudinal cooling channel; the first end of the cooling channel obtained within the shaft is connected to the delivery of a cooling circuit provided with a circulation pump and the second end, opposite to the first end, is connected to a cooling circuit inlet so that, in use, a stream of cooling fluid circulates through the cooling channel. However, such a solution is particularly complex and potentially not very reliable, because a passage for the cooling fluid needs to extend from a fixed part (the cooling circuit) to a movable part (the electric machine shaft) and vice versa. The passage of a cooling fluid from a fixed part to a movable part is particularly complex (and thus costly) to be made and always potentially exposed to the risk of leakage of cooling fluid.
In patent applications GB1283332A and US20060066156A1 it has been suggested to keep the longitudinal cooling channel of the electric machine shaft isolated and filled with cooling fluid; two heat exchangers are provided at the two ends of the shaft, which serve the function of removing heat from the shaft itself. In this case, the function of the cooling fluid present inside the cooling channel is to take the heat produced inside the rotor towards the ends of the shaft, where the heat is given to the heat exchangers. This solution has the considerable advantage of not needing a passage for cooling fluid that extends between a fixed part and a movable part, but on the other hand has a rather modest efficiency because the amount of heat carried by the cooling fluid present inside the cooling channel towards the ends of the shaft is relatively modest.
Patent U.S. Pat. No. 6,191,511B1 describes an asynchronous, liquid-cooled electric machine, which comprises a closed, liquid-cooling circuit which passes in sequence through the stator and the rotor shaft; a pump is incorporated in the rotor shaft and the cooling liquid is fed to the pump, by means of a static tube arranged axially inside the rotor shaft, and returns towards the stator passing between the rotor shaft and the static tube. The pump comprises a centrifugal pumping member, integral with a closed end of the rotor shaft, and longitudinal blades which are integral with the rotor shaft and are helical-shaped.